Fixes: #18436 - Prevent unassigning mac address when primary on an in…

…terface

netbox/dcim/models/devices.py

@@ -15,6 +15,7 @@
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext_lazy as _
 
+from core.models import ObjectType
 from dcim.choices import *
 from dcim.constants import *
 from dcim.fields import MACAddressField
@@ -1522,3 +1523,26 @@ class Meta:
 
     def __str__(self):
         return str(self.mac_address)
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+        # Denote the original assigned object (if any) for validation in clean()
+        self._original_assigned_object_id = self.__dict__.get('assigned_object_id')
+        self._original_assigned_object_type_id = self.__dict__.get('assigned_object_type_id')
+
+    def clean(self, *args, **kwargs):
+        super().clean()
+        if self._original_assigned_object_id and self._original_assigned_object_type_id:
+            assigned_object = getattr(self.assigned_object, 'parent_object', None)
+            ct = ObjectType.objects.get_for_id(self._original_assigned_object_type_id)
+            original_assigned_object = ct.get_object_for_this_type(pk=self._original_assigned_object_id)
+
+            if original_assigned_object and not assigned_object:
+                raise ValidationError(
+                    _("Cannot unassign MAC Address while it is designated as the primary MAC for an object")
+                )
+            elif original_assigned_object and original_assigned_object != assigned_object:
+                raise ValidationError(
+                    _("Cannot reassign MAC Address while it is designated as the primary MAC for an object")
+                )